Entamoeba histolytica is the pathogenic protozoan responsible for intestinal amebiasis and amebic liver abscess. E. histolytica is estimated to cause 50 million infections and 100,000 deaths annually and malnutrition is known to increase susceptibility to infection. The cytokine-like hormone leptin is reduced in the energy malnourished. Recent evidence that leptin is a pivotal mediator of susceptibility to amebiasis includes: 1) the identification of a polymorphism (Q223R) in the extracellular domain of the leptin receptor that significantly increases susceptibility in clinical studies; 2) demonstration that the Q223R polymorphism also increases susceptibility in mice; 3) the finding that tissue-specific deletion of the leptin receptor at the intestinal epithelium, but not in the hypothalamus, increases susceptibility in mice; and 4) demonstration that expression of the leptin receptor in single cells renders them resistant to amebic killing in vitro. Together these data indicate that leptin signaling directly protects intestinal epithelial cells from E. histolytica cytotoxicity, however the mechanism of protection is not understood. The central hypothesis of this project is that the Q223R polymorphism deregulates leptin receptor signaling and creates an intestinal epithelium that is permissive to E. histolytica infection. The following aims address the function of the Q223R polymorphism and the role of leptin signaling in cellular susceptibility to E. histolytica. Specific aim 1 will evaluate the functional consequences of the Q223R polymorphism by testing if leptin binding and/or surface expression of the leptin receptor is altered by the Q223R polymorphism. We will also test if the Q223R polymorphism alters activation of JAK2, SHP-2, STAT5 and STAT3 pathways through the leptin receptor. Specific aim 2 will determine the role of SHP-2, STAT5 and STAT3 signaling through the leptin receptor in cellular susceptibility to E. histolytica. The signaling pathway identified will be validated and downstream effectors of leptin receptor-mediated protection will be explored by microarray analysis. By evaluating the specific molecular consequences of a single polymorphism in the leptin receptor, we will begin to delineate a novel mechanism of mucosal immune defense. This work may have implications for the pathogenesis of other enteric pathogens, irritable bowel disease and intestinal cancer. Additionally, a greater understanding of the signaling events that connect malnutrition and the immune response will positively impact the health of children worldwide.